Places of learning once available to all (6)
I believe the answer is:
unisex
'available to all' is the definition.
(eg unisex toilets)
'places of learning once' is the wordplay.
'places of learning' becomes 'unis' (universities).
'once' becomes 'ex' (prefix meaning former or onetime).
'unis'+'ex'='UNISEX'
(Other definitions for unisex that I've seen before include "good for both partners" , "Suitable for male and female" , "Common to both genders, say clothes" , "like St Hilda's now, > but not St Trinian's!" , "for boys and girls" .)